Flying-seed-like liquid crystals

Kazuchika Ohta, Tomoyuki Shibuya and Masahiro Ando

Abstract:

In 1911, Vorlärnder reported liquid-crystalline properties of unique flying-seed-like alkali metal carboxylates. They are neither rod-like nor disk-like. Moreover, they have neither a rigid core nor flexible long chains. The molecular shapes are totally different from about 93000
general liquid crystalline compounds. These flying-seed-like liquid
crystals have been forgotten for about 100 years. We revealed and
established for the first time from precise microscopic observations
and temperature-dependent X-ray diffraction studies that sodium
diphenylacetate exhibits a hexagonal columnar mesophase, and that
potassium diethylacetate and potassium dimethylacetate exhibit a
smectic A mesophase. These old and new flying-seed-like liquid crystals will become a novel field of science.

Introduction:

In the East, there is a proverb called Wen Gu Zhi Xin (On Ko Chi Shin). This implies that to investigate old things is to find new things. In 1888, the first liquid crystals were discovered by Reinitzer. Since then people believed for many years that liquid crystalline compounds were rod-like molecules. In 1977, Chandrasekhar et al. discovered that disk-like molecules also show liquid crystalline properties.
Accordingly, in recent years people believe that liquid crystalline
molecules are rod-like as chopsticks or disk-like as dishes. Actually, there are few exceptions. To date, about 93000
liquid crystalline compounds have bee reported. Generally, each of the
liquid crystalline molecules consists of a strip-like or disk-like core
in the centre as the rigid part and two or more than six molten long
alkyl chains in the surroundings as the soft part.

Recently we noticed that about 100 years ago, in 1911, Vorländer reported some liquid crystalline compounds which are totally out of the general category mentioned above. This liquid crystalline compound has neither a rigid core nor a
flexible long alkyl chain. Moreover, the molecular shape is neither
rod-like nor disk-like but flying-seed-like.
The photograph shows maple seeds which are abundantly seen in Shinshu
University Ueda Campus from autumn to winter. They blow off from the
branches and fly by rotating. Various kinds of flying seeds are seen
world-wide and are very familiar to many people. We wondered whether
such unique-shaped molecules really exhibit liquid crystalline
properties, and what type of mesophases they show.

In the past 100 years, these flying-seed-like liquid crystals have seldom or never been investigated. As far as we know, there have been only three papers which were reported by Demus et al. and Sanesi et al. about 30 years ago.

Thus, we have established for the first time from temperature-dependent X-ray diffraction studies that sodium diphenylacetate (A2-Na) exhibits a hexagonal columnar mesophase (Colh), and that both potassium diethylacetate (B2-K) and potassium dimethylacetate (C2-K) exhibit a smectic A mesophase. These salts are flying-seed-like
and completely different from general molecular shapes of all the
conventional liquid crystalline materials, because their molecular
shapes are neither rod-like nor disk-like, and because they have
neither flexible long chains nor a rigid flat core. We believe that
these old and new flying-seed-like mesogens will become a novel field of science.